The very fact that PTs pretty much come to a stop when the treadle is in "neutral" atests to the point you make JJ.
One reason this does not constitute a real "emergency brake" however is it depends on everything upstream to remain intact and for there to be practically no internal leakage within the wheel motors. The valve you mention would address the issue of a hose or componant upstream from the valve failing, but there is more internal leakage in hyd motors, especially after much usage, than you might think. You would still slow down to 1/2 or 1 MPH, but it would not stop or hold you on a slope.
User solutions?
One would be to replace the OEM wheel motors with models that have internal wet-disc brakes. Most likely no hyd changes would be needed, but the motors will be longer and thus may need to be spaced out from the tractor body. This may be more of an issue in the rear tub than up front, however. This would give more stability at the expense of greater overall width, but no additional strain on wheel bearings as the off-set would be the same. Could just do one set if motors were otherwise identical.
Another possible solution would be to add one or two pairs of external brake discs and "spring apply/pressure release" calipers like the 18** series use and set up like the other wet-disc machines but without the hand pump. Even if done only to one end, such as the rear, you would have a fail-safe emergency & parking brake that could be used while in motion.
None of these options are inexpensive, but are certainly feasable. If PT at least offered such as a factory option, would be much more reasonable cost.
Just my $.02
BTW, JJ, I have a couple extra brake parts from when I changed out my wheel motors that "may" fit or be adaptable to your older 1445 if interested.
